Impulsive Hard X-Ray and Ultraviolet Emission during Solar Flares

Impulsive solar bursts of hard X-rays observed by the satellites OGO-1, 3, and S were studied in comparison with the concurrent enhancement of 10-1030 A radiation observed by means of sudden frequency deviations (SFD). The average ratio of the maximum enhancement of the impulsive flux at 10-1030 A to the maximum energy flux of X-rays at photon energies > 10 keV is found to be 10 . The ratio decreases with increasing central meridian distance (CMD) of the Ha flare. This effect is believed to be due to the increase in extreme-ultraviolet absorption by the solar atmosphere with the increase in CMD of the flare. The spectrum of the energetic electrons in the X-ray source region deduced from the observed X-ray spectrum indicates that the total kinetic energy in these electrons is comparable to that required to explain the enhancement of the 10-1030 A flux. Our interpretation of the observations indicates that the principal mechanism for the impulsive emission of 10-1030 A radiation during some hard X-ray events is the free-bound, bound-bound, and bremsstrahlung emission from the quasi-thermal plasma produced by the collision loss of energetic (>10 keV) electrons in the partially ionized solar plasma of density >1012 . These energetic the common link between the impulsive hard X-ray, extreme- ltraviolet, and microwave emission during solar flares.